hi hun
I'm not sure of any photographers you could use as I'm not in the same part of the country as you and mine were local not national photographers. As to the specifics though, as far as I know most places charge separately for the actual 'photo sitting' itself, then the pictures you purchase are additional costs (unless you manage to get a deal). Sittings can be anything from about £20-40 I think.
Once you've had your photos taken you'll be invited back to view them and shortlist your favourites, then make an order, if you want to. In terms of quality of the prints it really does depend on the photographer you use and how good they are. The charges by the photograhy place we used were £25 for a 7x5", £30 for an 8x6", these were the cheapest and they just increase in price the bigger the photo print is. Some offer packages deals and some don't.
I didn't find myself that there was any pressure to buy but I've heard that someplaces can be intense in trying to get you to buy as many prints as they can, but again you could go and get a feel of what they are like before you book.
And once you've had the sitting there's no commtiment to buy, you could walk out without purchasing any, but obviously if you've paid to have the sitting in the first place it's most likely you're going to order at least one print. We got a deal which was £20 for the sitting and a 7x5". We ended up buying an additional different photo that we liked for £30, but we knew we weren't going to spend a lot before we attended the viewing, so if you do go for it, try to have a spend limit in mind.
Hope that helps a bit
Edit; on another note, Max Spielmann do a great photo set for £5.99 where you get several different size prints of your chosen photo. They have a backdrop sheet which LO sits against so it looks professional. Not too sure how they compare to prof photos, but my cousin had them done of her son and they were quite impressive for the price.